Sailing craft of one sort or another have been used for movement across the seas throughout recorded history. For hundreds of years, all great navies and merchant fleets of the world consisted of sailing vessels. By the early 1900's, however, steamships had almost completely replaced sailing vessels for military and commercial purposes. The development of sailing as a sport began when sailing ships declined in commercial importance.
Most sport sailboats today are Marconi-rigged, with triangular sails having a vertical edge or luff along a mast and a lower edge or foot secured to a boom. The mainsail is fastened to the back of the mainmast. Smaller sailboats, such as catboats, may have only the single mainsail. Other, larger craft may have a jib extending forwardly of the mast, as in the case of a sloop, or may have additional masts, as with yawls, ketches or schooners.
Recently, a great deal of interest has developed in sailboards (also called "windsurfers") wherein a single mast is mounted on a surfboard by means of a flexible universal joint, with a single sail having a forward edge lying along the mast. In sailing a sailboard, the person stands on the board next to the sail, holding onto an oval boom (called a "wishbone") surrounding the mast and sail. The sailboard is controlled by a combination of body movement along the board, body lean and sail positioning.
With small sailboats, the sail is an approximate right triangle, with the lower edge substantially perpendicular to the mast and parallel to the boat deck. With sailboards, the lower sail edge generally angles roughly 45.degree. to mast and board. With these small craft, it is important that the single sail provide the maximum aerodynamic effect to provide optimum efficiency and speed. The design of sails, masts, rigging, etc. has been the subject of intense development and innovation for hundreds of years. Many attempts have been made to improve the arrangement and shape of the sail which has always been a single thin sheet of woven material.
From time to time, depending upon wind condition, sailboards and small sailboats may capsize. Generally it is possible for the sailor alone to right such small craft. However, if the sail goes below the water surface, or conditions are poor, the weight of the wet sail and any water over it may make righting the boat or board very difficult or impossible.
From its obscure beginnings in the late sixties, sailboards have become the fastest sailing vessels in the market today. Recorded sailboard speeds have made a dramatic increase from the top speed of 19.1 knots in 1977 to over 38 knots in 1986. The early gains in speed can be attributed to smaller and lighter hulls which were made possible by the extensive use of advanced materials such as graphite, Kevlar, the carbon with epoxy resin. More recent speed improvements, however, were provided by better sail designs.
Sailboard sails have several especial design features not normally associated with conventional sails. As a basic requirement, these sails have to be light enough to able to lifted out of the water by one individual without any mechanical devices such as winches or blocks. They need to be able to withstand not only high wind forces but also hydrostatic loads. Furthermore, these sails have to be able to sustain dynamic wave loads should they be used under surf conditions.
The basic form of sailboard sails have evolved from catamaran main sails. Early sailboard sails were triangular in shape with few or no battens. Today, practically all high performance sails have full battens. The so called RAF (Rotating Asymmetrical Foils), Camber Induced and Double Luff sails are typical examples. The differences between these sails is simply in the method by which they try to control the shape of the sail's leading edge.
Traditional single sheet sails provide a very high lift coefficient--higher than wingsails (like airplane wings). However, in high-speed, high-wind conditions such as often encountered by sailboards, they create a much higher drag which, in turn, inhibits greater speed. Dramatic improvements in speed can only be achieved by reducing this drag. Wingsails offer a viable solution to this problem.
Rigid wingsails have been tried on catamarans and sailboards before but they have not been very successful. Their internal structure as well as their double panel construction makes them too heavy particularly for sailboards. Their built-in rigidity does not allow them to twist and depower when a sudden gust or change in wind direction occurs, making them hard to control. They are also extremely difficult to transport since they cannot be rolled or folded. Furthermore, they have to be symmetrical in cross-section so they can perform well in either tack. Such a section is less efficient than an asymmetrical one.
By using inflatable sails, most of the disadvantages of rigid wingsails can be minimized or eliminated while its high lift-to-drag ratio can be maintained or even surpassed. An inflatable sail's unique ability to vary its camber and to assume an asymmetrical shape makes this possible.
Inflatable sails are disclosed by Cochran in U.S. Pat. No. 3,298,346 and Birchill in U.S. Pat. No. 3,391,668. In each of these, the sail itself is made up of two panels which are sealed to permit inflation. Making the sail itself airtight is very difficult, requiring very high strength-to-weight materials and requiring bonding and sealing methods different from the conventional sewing of sail panels together. Normal sailcloth is not airtight and normal plastic air impervious materials do not have sufficient strength for use as sails. Inflated sails of such normal plastic materials would be likely to leak and distort.
Cochran requires a number of transverse webs within the inflated sail to maintain sail shape. Although the webs tend to resist changes in sail shape in response to changes in wind direction and velocity the sail panels between the webs will assume a ripple-like configuration in the vertical direction. When the sail is raked forward or aft as is necessary in sailboards to control direction, the ripple-like configuration changes the sail's horizontal cross-section to a less effective one. This has a detrimental effect on performance.
Birchill inflates portions of the sail, a vertical area near the mast providing a bulge between mast and the flat balance of the sail. The sail is inflated by a pair of air scoops at the top of the sail, facing forward. These air scoops would simply fill up with water when used with sailboards, thus making it impossible to raise the sail.
Penduff in French Pat. No. 2,518,956 discloses a sail having two panels extending aft from an airfoil leading edge-shaped mast. A narrow vertical inflated bag is positioned well behind the mast. Battens are provided in both sail panels to control sail shape. This a a heavy and cumbersome arrangement, ill suited to sailboards which use round masts and require lightweight sails to permit righting by one person. Without the battens concave areas would certainly form between the mast and the inflated bag, on the windward side of the sail, resulting in a very poor airfoil shape.
Thus, there is a continuing need for improved sails that combine low weight and simplicity with the high lift-to-drag ratios typical of wingsails without their inherent weight problems.